Go outside on a fine day and find one of those small white clouds. Not the flat ones that cover the whole sky — the round kind with clean edges that sits alone in the blue and looks like it has no weight at all.
It weighs about 500,000 kilos.
That is 100 elephants. A hundred full-grown elephants, in a line, standing on nothing, 1 kilometre above the street, and none of them coming down.
People have counted it, more than once. A cloud of that kind is about 1 kilometre across, 1 kilometre deep and 1 kilometre high, and you can count how much water sits in one small box of it and then count the boxes. The answer comes out near 500,000 kilos, every time.
So why does it stay up there?
The first answer is the size of the water. The cloud is not one great weight of water; it is water cut into pieces so small that you cannot see a single one of them. It takes about 1,000,000 of these little drops to make one drop of rain.
Now, a drop that small does fall. It has weight, and nothing holds it. But it falls slowly — as far as your arm is long, in about a minute. Everything that falls through air is fighting the air, and the smaller a thing is, the more the air wins.
And under the cloud, the air is not still. The ground gets warm in the sun all day, and the warm air above it rises. It rises faster than those drops can fall. So the drops go down, and the air pushes them back up, and the cloud stays exactly where it is. It has been falling all day and it has not arrived anywhere.
The second answer is stranger, and it is the one people never guess.
The cloud is heavy, but the air is heavier.
That same 1 kilometre box of sky holds about 1,000,000,000 kilos of air — more than 2,000 times the weight of the water inside it. We forget that air has weight at all, because we live down inside it. But it does, and a cloud sits in the air like a boat sits in water. A boat is heavy too. It stays up because the thing under it is heavier.
Then, one afternoon, it rains. What changed?
The drops met each other. Inside the cloud the air is moving all the time, and drops keep running into other drops and joining them. A bigger drop falls faster. A faster drop catches more drops on the way down, which makes it bigger again, which makes it faster again — and after a drop has joined about 1,000,000 others, no rising air can hold it any more.
That is rain. Nothing new arrived. The water that lands on your head this evening was over your head all afternoon. It was simply in the wrong shape to come down.
Look up once more before you go in. The cloud has not moved much. It looks light, and it looks empty, and neither of those things is true.
There are 100 elephants of water over the street, and the children under them are quite safe — for now.